The present invention relates to a system for sensing voltage and current values within a power cable, and more particularly to a system for sensing electrical energy usage and accurately communicating information regarding such electrical energy usage to a remote location.
Many electrical devices currently exist for measuring the voltage, current and power carried by a power cable. Typically, a sensing transformer in the form of a toroidal coil of wire wrapped on a magnetically permeable core encircling the power cable is used to sense the electrical energy within the power cable. A split toroidal core is typically used because it can be easily affixed to an existing power cable without disconnecting the power cable. The wire of the coil is electrically connected to an electronic circuit which produces an output representative of one or more factors of the electrical power carried in the cable.
Bullock, U.S. Pat. No. 5,066,904 discloses a current sensor using a sensing coil having a toroidal core for measuring large magnitudes of AC current within a power cable. A shunt parallel with the power cable passes through the center of the toroidal core of the coil. Alternating load current in the shunt induces a voltage in the wire windings around the core, and the induced voltage is used as the input to an operational amplifier.
Miller, U.S. Pat. Nos. 4,413,230 and 4,491,790 disclose different electrical energy meters, each of which includes a transducer that produces output signal voltages, e.sub.i and e.sub.v, respectively representative of the current and voltage components of the electrical energy in a power cable. The signal voltages are then used as input signals to a closely associated power measuring circuit. The measuring circuit of each of the Miller patents is in close proximity to the power cable and transducer, and as a result, attenuation of the signal voltages between the transducer and the measuring circuit is insignificant. However, if the same voltages were applied to a relatively long transmission line from the transducer to a remote measuring circuit, the signal voltages would be significantly attenuated, thereby significantly decreasing the accuracy of the measurements obtained.
Swarztrauber, et al., U.S. Pat. No. 4,783,748 discloses a system using a plurality of sensing units at remote locations for determining power within a plurality of power cables. A digital two-way communication system links the sensing units with a central processing unit that performs calculations on the signals produced by the remote sensing units. However, the use of a digital two-way communication system requires expensive circuitry to encode the signal to be produced by the remote sensing units and decode the signal received at the central processing unit.
Fernandes U.S. Pat. Nos. 4,794,327 and 4,855,671 disclose sensing power within multiple remotely located power cables and transmission of radio frequency signal representative of the power in each power cable to a central receiver for processing. Fernandes, however, requires expensive signal processing circuitry at both the transmitting and receiving locations, in addition to the further expense of transmitters and a receiver.
Current transformers with "air core" coils may be used in electrical measurement systems. An air core coil is a coil of wire wrapped on a core of material (e.g., plastic) that is not magnetically permeable. Air core coils provide a more linear response to substantial variations in load current levels and variable frequencies than can be obtained with coils wrapped on magnetically permeable cores, thereby permitting more accurate measurements of power carried in an electrical cable. However, air core coils inherently produce low induced voltages, as compared to coils wrapped on magnetically permeable cores. Low induced voltages require a metering circuit responsive to such low voltages to produce accurate results.
Howell, U.S. Pat. No. 4,297,741 discloses the use of air core current transformers to produce output voltages proportional to load currents in respective power cables. A full-wave rectifying network is connected to the current transformer secondaries to provide a rectified output voltage signal. A comparator issues a circuit breaker trip signal when the output voltage signal of the rectifying network achieves a preselected threshold voltage. Such a voltage signal, however, is susceptible to AC interference and to attenuation with transmission more than a few feet.
May, U.S. Pat. No. 5,003,278 discloses a pair of housing halves for a high frequency noise suppressor which can be snapped around a cable in clamshell fashion. A plurality of flexible fingers extend radially inward from the halves to engage a cable within a central opening. The fingers can bend outwardly to accept cables of various sizes. May, however, does not include structure within the housing for holding electronic circuitry.
What is still needed, then, is an inexpensive system for sensing power use and providing accurate signals indicative of power used in remotely-located power cables, over wide ranges of power and at various AC frequencies, and for conducting such signals to a remote location for interpretation and use of the information from the signals.